1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device such as a nitride semiconductor laser device.
2. Description of Related Art
Nitride semiconductor laser devices that lase in an ultraviolet to visible region of the spectrum have been fabricated on a commercial basis as well as on a trial basis by the use of nitride semiconductor materials exemplified by GaN, AlN, InN, and mixed crystals thereof. For this purpose, GaN substrates are typically used, the use of which, however, has thus far never led to the production of nitride semiconductor laser devices with satisfactorily long lasing lives. Thus, longer lasing lives are sought after. The lasing lives of nitride semiconductor laser devices are affected by cracks and the like that develop when nitride semiconductor layers are laid on top of a GaN substrate. Thus, the development of cracks is one of the factors that cause the deterioration of nitride semiconductor laser devices, and therefore needs to be minimized.
According to one conventionally proposed technique against the above problem, a nitride semiconductor device is fabricated by growing on top of a GaN substrate a nitride semiconductor having a lower thermal expansion coefficient than GaN and then forming further on top thereof a semiconductor light-emitting device structure. This helps reduce the development of minute cracks in particular in the n-type contact layer formed on top of the GaN substrate (see Japanese Patent Application Laid-Open No. 2000-299497, hereinafter referred to as Patent Publication 1).
However, even when a semiconductor laser device structure is laid on top of a GaN substrate according to the technique proposed by Patent Publication 1 mentioned above, cracks do sometimes develop on the surface of the wafer. These cracks that have developed on the surface become one of the factors that cause the deterioration of the characteristics of semiconductor laser devices, leading to unduly short lasing lives, and to lower yields as result from chips being divided in an unexpected manner due to clefts developing along cracks.